Okay, I'll take a look at this
It reminds me of an advanced Octopus so you aren't grabbing me with originality. The name doesn't grab me. A Horror in the Waves sounds like it should be an Aberration and not a Magical Beast. That said, let's get to the meat of this thing. It is more powerful than you originally thought.
Type: Magical Beast (Aquatic). This means that it cannot breathe air unless it also has the Amphibious special quality.
Hit Dice is missing bonus hp from Con. It gets 36 bonus hp from it's Con and HD.
Hit Dice: 18d10+36 (135 hp)
Speed: you gave it a fly speed but did nothing with it. It feels tacked on. You don't mention how it flies when it attacks and it has no feats for flying creature. Flight isn't something that you just "give" to a creature it should be part of of its anatomy like having wings. I'd recommend losing the flight speed since it can't breath out of water as it doesn't have the Amphibious special quality as I stated earlier.
BAB is + 18 because it is a Magical Beast. It gets Full BAB just like a Fighter.
Grapple is +36. (BAB +18, Size +8, Str +6, racial +4l)
Attack numbers are wrong: BAB + Str + Size = 18 + 6 -2 for a total of +22
Attack: Tentacle +22 melee (1d8+6 plus dreadful strike)
Full Attack: 8 tentacles + 22 melee (1d8+6 plus dreadful strike) and bite +20 melee (1d6+3 plus dreadful strike)
List your special abilities and special qualities in alphabetic order so you don't forget something. Jet is missing from the list.
You have the wrong number of feats. Bonus feats don't count towards the full number of feats. It should have 7 feats plus bonus feats. Multiattack and Power Attack aren't bonus feats for this creature. It has enough HD that it qualifies for those feats. It you want to give it another feat, that feat would be the bonus feat.